Prediction of elastic-plastic fatigue crack growth rate and crack closure under varying loadings

Load-controlled fatigue crack growth tests were carried out on two kinds of steels, S35C and SM50A, under constant amplitude loading and repeated two-step loading over a wide range from the linear elastic region to the post-yield one, and crack closure was investigated by using the minicomputer-aided unloading elastic compliance method. The fracture mechanics parameter, ΔJ/ E(1-Jmax/C), was found to be a good parameter in expressing the fatigue crack growth rate under constant amplitude loadings irrespective of the stress ratios and materials. It was found that the constant, C, was not related to the elastic-plastic fracture toughness, JIC, but to the fracture toughness. The fatigue crack growth rate under repeated two-step loadings where both one-directional and cyclic plastic deformation were observed remarkably, could be well predicted by the linear summation rule of crack growth by using ΔJ*/E(1-Jmax/C), where ΔJ* is the J integral range-pair taking account of one-directional deformation under low-level loadings.